1. Field of the Invention
This invention relates to materials for dental prosthetic devices such as tooth fillings. In particular, this invention relates to a beryllium-free nickel-chromium-tungsten alloy further comprising silicon and aluminum. The alloy of this invention bonds well to porcelains commonly used in dental restorations.
2. Brief Description of the Prior Art
Fixed and removable dental prostheses such as crowns, artificial teeth, and bridges have traditionally been made using alloys of precious metals such as gold or palladium. Such alloys bond well with porcelains and resin-base composites. However, given the increasing expense of precious metals, substitute materials have been sought for years.
Over the last fifteen years, nickel-chromium alloys containing less than two percent beryllium by weight have emerged as successful alternatives based on the alloys' unique mechanical and thermal properties. Such nickel-chromium alloys have a higher modulus of elasticity than precious metal alloys, are light in weight, and have a low thermal conductivity. These alloys currently occupy a high percentage of the dental restoration market. However, because of concerns over the allergenic and toxicity potentials of beryllium, some consider such alloys to be health hazards, and the use of beryllium is becoming disfavored. Certain countries have gone so far as to pass regulations banning the use of beryllium in dental alloy materials. Beryllium-containing alloys also exhibit inferior corrosion resistance in acidic environments (lower pHs) compared to alloys that do not contain beryllium.
A number of patents describe beryllium-free nickel-chromium alloys, for example U.S. Pat. No. 3,914,867 to Manning et al.; U.S. Pat. No. 3,896,547 to Kulwiec; U.S. Pat. No. 3,841,868 to Dudek et al; U.S. Pat. No. 3,948,653 to Tesk et al.; U.S. Pat. No. 4,053,308 to Tesk et al.; U.S. Pat. No. 4,114,272 to Saragossi; U.S. Pat. No. 4,210,447 to Tsci and U.S. Pat. No. 4,243,412 to Tandon, as well as German Patent No. 2713755 to Rademacher. Many of the nickel-chromium alloys based on the above patents were introduced in the market and used by the dental profession with varying degrees of success. However, none of these alloys have successfully replaced beryllium-containing alloys.
In fact, the majority of these beryllium-free alloys have been withdrawn from the market because of their limited success. Such products have likely failed due to their poor castability, and the green color of the oxides formed on the alloys' surfaces. It is also likely difficult to control the amount of oxide formation, thereby resulting in poor adhesion and inferior mechanical properties. Accordingly, there remains a need for a dental alloy with the advantageous properties of beryllium-containing nickel-chromium alloys, but that will avoid the health hazard concerns associated with these alloys.